The naked marine dinoflagellates Cochlodinium convolutum and C. helicoides are reported for the first time in the Gulf of California. They occurred in seawater at 21-24.5°C. Cochlodiniumconvolutum was found in both bottle and net samples; its abundance ranged from 400-2200 cells L_1. A total of 96 specimens of C. convolutumand 23 specimens of C. helicoides were examined from the net phytoplankton samples. A short description is given for each species, and photomicrographs provide information about the main morphological characteristics.